To gain further insight into the pathogenesis and immunological factors involved in Marek's disease (MD), transplantable Marek's disease virus (MDV) induced lymphomas will be studied in inbred strains of chickens. Various parameters concerning the identification of MDV transformed cells during in vivo serial passage will be examined. Antigenic make-up and immune response to MD tumors during successive passage and differences between tumors induced by MDV strains of low and high virulence, with regard to transplantability and cellular composition, will be analyzed. Possible differences in turkey herpesvirus (HVT) vaccination effects, as related to immunogenicity of transformed cells, will be examined to determine whether HVT vaccination induces immunity to transformed cells of differing antigenicities or protects at a level prior to malignant transformation and cell proliferation. An immunologic and histopathologic analysis will be made to explore the basis for genes at the major histocompatibility complex controlling MDV-induced transient paralysis.